Ferdinand II (9 July 1578 – 15 February 1637) was Holy Roman Emperor, King of Bohemia, Hungary, and Croatia between the late 1610s and his death in 1637. He was the son of Archduke Charles II of Inner Austria, and Maria of Bavaria. His parents were devout Catholics, and, in 1590, they sent him to study at the Jesuits' college in Ingolstadt, because they wanted to isolate him from the Lutheran nobles. In July that same year (1590), when Ferdinand was 12 years old, his father died, and he inherited Inner Austria–Styria, Carinthia, Carniola and smaller provinces. His cousin, the childless Rudolf II, Holy Roman Emperor, who was the head of the Habsburg family, appointed regents to administer these lands.

Ferdinand was installed as the actual ruler of the Inner Austrian provinces in 1596 and 1597. His cousin Rudolf II also charged him with the command of the defense of Croatia, Slavonia, and southeastern Hungary against the Ottoman Empire. Ferdinand regarded the regulation of religious issues as a royal prerogative and introduced strict Counter-Reformation measures from 1598. First, he ordered the expulsion of all Protestant pastors and teachers; next, he established special commissions to restore the Catholic parishes. The Ottomans captured Nagykanizsa in Hungary in 1600, which enabled them to invade Styria. A year later, Ferdinand tried to recapture the fortress, but the action ended in November 1601 with a defeat, due to unprofessional command of his troops. During the first stage of the family feud known as the Brothers' Quarrel, Ferdinand initially supported Rudolph II's brother, Matthias, who wanted to convince the melancholic Emperor to abdicate, but Matthias' concessions to the Protestants in Hungary, Austria, and Bohemia outraged Ferdinand. He planned an alliance to strengthen the position of the Catholic Church in the Holy Roman Empire, but the Catholic princes established the Catholic League without his participation in 1610.

Philip III of Spain, who was the childless Matthias' nephew, acknowledged Ferdinand's right to succeed Matthias in Bohemia and Hungary in exchange for territorial concessions in 1617. Spain also supported Ferdinand against the Republic of Venice during the Uskok War in 1617–18. The Diets of Bohemia and Hungary confirmed Ferdinand's position as Matthias' successor only after he had promised to respect the Estates' privileges in both realms. The different interpretation of the Letter of Majesty, which summarized the Bohemian Protestants' liberties, gave rise to an uprising, known as the Second Defenestration of Prague on 23 May 1618. The Bohemian rebels established a provisional government, invaded Upper Austria, and sought assistance from the Habsburgs' opponents. Matthias II died on 20 March 1619. Ferdinand was elected Holy Roman Emperor on 28 August 1619 (Frankfurt), two days before the Protestant Bohemian Estates deposed Ferdinand (as king of Bohemia). News of his deposition arrived in Frankfurt on the 28th but Ferdinand didn't leave town until he'd been crowned. Bohemia offered their crown (King of Bohemia) to the Calvinist Frederick V of the Palatinate on 26 August 1619.

The Thirty Years' War began in 1618 as a result of inadequacies of his predecessors Rudolf II and Matthias. But Ferdinand's acts against Protestantism caused the war to engulf the whole empire. As a zealous Catholic, Ferdinand wanted to restore the Catholic Church as the only religion in the Empire and to wipe out any form of religious dissent. The war left the Holy Roman Empire devastated and its population did not recover until 1710.